Clubs - some may be run by the school and be cheaper, or run by outside providers which typically cost more. For example, our prep had running club for £25/term, and other clubs were £70-150 per term. I think Lambda for drama was quite expensive (we didn’t do that so can’t recall the actual price).
Music lessons (optional, instrument specific) were £300 per term for a 30 min lesson per week.
Residential trips in UK ranged from £400 in year 4 to £660 for year 6.
Optional ski trip in year - £1600 (included flights, ski school, equipment, accommodation, meals, other activities off the slopes).
Each year our class reps would ask for a donation to cover some PTA organized activities, staff gift cards for Christmas, non-uniform day donations etc. Think this was around £30/year per child.
Other things that might come up is the ask of donations for raffle prizes for events the school or PTA hosts - like for a summer fair, food for harvest festival collections/charity etc. as well as buying raffle tickets, or tickets for entry to optional PTA organised events (quiz, parents drinks etc).
School uniform - try to find out how much is “branded”/must be bought at specialist shop (or sencond hand sale) vs non-branded can be bought at cheaper stores. Definitely take advantage of second hand uniform sales. Our prep ran them each term and had loads of stuff, a lot in decent condition - have to get in early though to get the good stuff!